On 5/11/19 3:41 AM, Hadley Wickham wrote:
For what it's worth, I don't think this strategy can work in general,
because a class might have attributes that depend on its data/contents
(e.g. https://vctrs.r-lib.org/articles/s3-vector.html#cached-sum). I
don't think these are particularly common in practice, but it's
dangerous to assume that you can restore a class simply by restoring
its attributes after subsetting.
You're probably right that there are lurking perils in general, but I am
not trying to "restore a class". I simply want to *retain* attributes
of columns in a data frame.
* I have a data frame X
* I attach attributes to certain of its columns;
attr(X$melvin,"clyde") <- 42
(I *don't* change the class of X$melvin.)
* I form a subset of X:
Y <- X[1:100,3:10]
* given that "melvin" is amongst columns 3 through 10 of X,
I want Y$melvin to retain the attribute "clyde", i.e. I
want attr(Y$melvin,"clyde") to return 42
There is almost surely a better approach than the one that I've chosen
(isn't there always?) but it seems to work, and the perils certainly are
not immediately apparent to me.
cheers,
Rolf
--
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276
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